The plague is an old microbial foe that has haunted our cities and our ports for millennia, killing millions of people in waves of pandemics since antiquity. But Yersinia pestis no longer has the same presence, or stranglehold, in our society and seems negligible when we consider the current state of microbial affairs - increasing levels of antibiotic resistance and novel and emerging viral pathogens, just to name a couple of today's most pressing issues. Even its moniker, “the plague," has been appropriated for more contemporary microorganisms that appear to come from nowhere and quickly, fatally sweep through a population - SARS and HIV are prime examples of two new "plagues."
A female Xenopsylla cheopis flea, known as the “oriental rat flea,” one of the major vectors for transmission of the bacterium Yersinia pestis, the causative agent of plague. Image: CDC/ World Health Organization. But the golden oldie is still alive and well in our world and, according to a recent review from the American Journal of Tropical Medicine and Hygiene, still up to its old tricks. Author Thomas Butler looks at the current shape of Yersinia pestis and asks what this old adversary of ours is doing now. What's the scoop on plague in the 21st century? Is "ring-around-the-rosie" still relevant in this modern era of twerking? What Butler finds in his review on the modern plague is that this ancient disease "continues to surprise us with first-ever events." Over the past century, Y. pestis’ presence in society has been largely defanged thanks to widespread improvements in sanitation and urban hygiene. The bacteria is a zoonotic disease, reliant on fleas feasting on various rodent species spreading the infection to humans through bites. Plague is largely absent in developed societies thanks to housing and infrastructures that hinder rodent infestations and delimit exposure to fleas as they jump from their rodent host to a human one. But in places where those luxuries are less prevalent, plague thrives within various rodent species including shrews, gerbils, prairie dogs, squirrels and voles. The bacteria has established an endemic presence in Africa, where cases of the infection comprise over 97% of the world’s 21,725 cases in the first decade of the 21st century. The Congo takes the lead in highest numbers of cases, largely owing to its widespread social destabilization from years of civil war and a concomitant breakdown in public infrastructures. The shock value of plague lies in its infamous role in large scale outbreaks that slashed populations - the Plague of Justinian in 541 AD and the Black Death are perhaps the two most well-known historical events - but the norm for this ancient bacteria now seems to be less about massive outbreaks sweeping through populations and increasingly characterized by small, relatively isolated incidents of transmission among sub-communities within larger populations and individuals encountering the disease under less-than-mundane circumstances. A few illustrative examples of the sub-community phenomenon: Over the course of three months from December 2004 to March 2005, 136 men working in an open-pit diamond mine in the heavily forested region in the eastern region of Oriental Province Democratic Republic of Congo were infected with the most virulent form of the infection, pneumonic plague, and 57 miners succombed to the infection (2). In the Qinghai Province of China in 2009, 12 people were infected after a herdsman was exposed to plague from a sick dog. One high profile case in the United States occurred after a wildlife biologist conducted an autopsy sans gloves and face mask on a mountain lion at Arizona’s Grand Canyon National Park. The takeaway is not just that the susceptible population has narrowed in recent centuries - after all, virtually the whole of the human population was susceptible for much of history - but also that methods of transmission have grown more eclectic since the glory days of rats and fleas and rings around the rosie and what-have-you. Indeed, sometimes fleas, the classic culprit behind outbreaks of the plague, can be excluded from the equation altogether: outbreaks in Jordan and Afghanistan showed that exposure to meat from camels infected by rodent flea bites prior to butchering can transmit plague. In the Afghan outbreak, 83 people became ill and 17 died following contact with the meat of an infected camel. Isolated cases and small outbreaks are driven by our interactions with animals, even with our most beloved domesticated pets. Butler writes of the discovery that pet dogs in the United States are capable of plague transmission when they trot into our homes and sleep in our beds, all the while carrying rodent fleas. We now know that “sleeping in a bed with a pet dog [is] a risk factor for human plague.” It is this sort of series of cases and small outbreaks, from China to Afghanistan to Arizona, that serves as a reminder that this ancient pathogen can still haunt us, hiding in our backyards. Plague has adapted to our new modern world and the constraints imposed by durable housing and public health infrastructures; it now causes smaller outbreaks and isolated cases instead of sweeping pandemics, with unconventional vectors and via unexpected means. No longer a plague but as much a pest as ever. Reference T Butler (2013) Plague Gives Surprises in the First Decade of the 21st Century in the United States and Worldwide. Am J Trop Med Hyg. 89(4): 788-93